1. Field of the Invention
The present invention relates to an alternating/direct current (AC/DC) adaptor of a device, such as a portable computer, and more particularly to a power-saving circuit of an AC/DC adaptor for a device, such as a portable computer, capable of minimizing the power consumption of the AC/DC adaptor in a power-saving mode of the portable computer.
2. Description of the Related Art
In general, an AC/DC adaptor for a portable computer is a device which receives commercial alternating current (AC ) power, converts the AC power into a certain direct current (DC) voltage, and supplies the DC voltage to the portable computer. While the portable computer is widely used, technology relating to the AC/DC adaptor has rapidly advanced recently.
FIG. 1 is a view for explaining an AC/DC adaptor for a portable computer A. The portable computer A is connected by line B to an AC/DC adaptor C, the AC/DC adaptor C being connected by line B1 to a plug D which engages an AC external power source outlet E. In case of a portable computer using a thin film transistor liquid crystal display (TFT-LCD), a DC voltage of 14V is required in general, so the AC/DC adaptor C receives the commercial AC power supply of 110V or 220V, converts the AC power supply into a DC voltage of 14V, and supplies the DC voltage to the portable computer A.
FIG. 2 is a circuit diagram for showing a conventional AC/DC adaptor, such as AC/DC adaptor C of FIG. 1, for a portable computer, such as portable computer A of FIG. 1. The conventional AC/DC adaptor C for a portable computer in FIG. 2 is constituted by an input unit 1, a power transistor 2, a transformer 3, an output unit 4, a feedback unit 5, and a driver 6. Operation of the conventional AC/DC adaptor C for a portable computer A will now be described with reference to FIG. 2. First of all, a commercial AC power supply of 100V or 220V is applied to the input unit 1. The input unit 1 converts the AC power supply into a DC voltage through an LC filter of capacitors C1, C2, and an inductance L1, a bridge diode D1, and a smoothing capacitor C3, and supplies the DC voltage to the primary coil P of the transformer 3.
Continuing with reference to FIG. 2, at this time, if the power transistor 2 repeatedly switches on and off based on a pulse width modulation (PWM) control signal input from the driver 6, an induced electromotive force from the primary coil P to the secondary coil S of the transformer 3 is generated. An AC voltage output from the secondary coil S of the transformer 3 is converted into a certain DC voltage (V.sub.out) of, in general, 14V through a rectifying diode D2 and a smoothing capacitor C4 of the output unit 4, and the DC voltage is applied to a portable computer, such as portable computer A of FIG. 1.
Further, with reference to FIG. 2, also at this time, a feedback unit 5 is, in general, provided to the AC/DC adaptor C for maintaining the output voltage of the output unit 4 at a certain voltage. air That is, the feedback unit 5 produces a feedback voltage based on the output voltage of the output unit 4 and outputs the feedback voltage to the driver 6. The driver 6 compares the feedback voltage with a reference voltage, produces the PWM control signal, and supplies the PWM control signal to the power transistor 2, so that the power transistor 2 is switched on and off. Accordingly, the output voltage of the AC/DC adaptor C is maintained at a constant voltage all the time to be supplied to a portable computer, such as portable computer C of FIG. 1.
In general, if a user does not use a portable computer and the portable computer is also not turned off, the portable computer typically recognizes the state and reduces the power consumption of the portable computer itself to below a certain level, and, a display used by a user just before such power reduction typically appears automatically and instantly when the user wishes to again use the portable computer, differently from a turnoff of the power for the portable computer, as in the prior art. Such a portable computer having a power-saving function is realized with a display power management system (DPMS) according to a standard plan of the Video Electronics Standard Association (VESA), for example.
However, even though a portable computer carries out the power-saving mode, a conventional AC/DC adaptor for a portable computer is normally operated to supply a rating voltage to the portable computer, which can cause an unnecessary power loss in the AC/DC adaptor itself.